Cosmology is in crisis. The pillars of science鈥檚 most successful model of how the universe formed 鈥� and where it is heading 鈥� are starting to crack. One of those pillars is dark energy, a mysterious force causing the universe to expand faster and faster, defying what Einstein and others once expected. What is the true nature of dark energy, and how do we finally pin it down?

Physicist Tessa Baker at the University of Portsmouth, UK, is at the forefront of this search, working with some of cosmology鈥檚 most powerful tools, including the Laser Interferometer Gravitational-Wave Observatory (LIGO). By studying ripples in space-time sent out by the most extreme events in the cosmos, such as black hole mergers, experimentalists like Baker are beginning to test radical ideas: that dark energy might stem from hidden dimensions, elusive fields detectable only in deep space, or, Baker鈥檚 personal favourite, from the thermodynamic properties of space-time itself.

It is widely thought that our universe is expanding at an ever-accelerating rate. But could we have that wrong? That is what a group of scientists from South Korea claims, but other scientists have major concerns about the work.

Our universe has been expanding since the big bang 13.8 billion years ago. Several strands of evidence, including observations of distant dying stars called type Ia supernovae, have suggested that this expansion is accelerating. One of the main explanations for the driver of this acceleration is a mysterious force called dark energy, the discovery of which won the 2011 Nobel prize in physics.

at Yonsei University in South Korea and his colleagues now say this might be wrong. Type Ia supernovae are caused when the remnant core of a star like our sun, known as a white dwarf, explodes in a binary system. Astronomers use these 鈥�standard candles鈥� as trustworthy measurements of distance across the cosmos because they are thought to be uniformally bright.

But Lee and his team say the brightness varies strongly with the age of the stars, based on their analysis of 300 host galaxies. They say that distant supernovae may appear to be fainter than expected and this is usually put down to the accelerating expansion of the universe, but, once this 鈥渁ge bias鈥� is taken into account, the accelerating expansion disappears.

Instead, Lee says their findings suggest the expansion of the universe began decelerating 1.5 billion years ago, and could even reverse in the future, a scenario called the 鈥渂ig crunch鈥� in which the universe could end in a reverse big bang. Previously, he says, 鈥渁 big crunch was out of the question. But now it is a possibility.鈥�

at the Space Telescope Science Institute in Maryland, one of the recipients of the 2011 Nobel prize in physics, disagrees with that claim, pointing to earlier work by the group in 2020 that had been refuted. 鈥淭he same group鈥檚 new work repeats the argument with little change,鈥� he says, noting that making measurements of stellar ages for type Ia supernovae at large distances is very difficult. He says Lee鈥檚 team used a mean stellar age derived from the host galaxy. 鈥淭he theory behind this is weak because of a lack of certainty about how the [star] forms,鈥� says Riess.

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There are known issues with how age affects the brightness of type Ia supernovae across the universe, says at the University of Southampton, UK, but these are already accounted for in measurements of dark energy. 鈥淚鈥檓 very sceptical this will lead to a decelerating universe,鈥� he says.

Upcoming observations with the Vera C. Rubin Observatory in Chile are expected to greatly expand the number of known type Ia supernovae in the universe, from the thousands catalogued today to tens of thousands. That will allow us to 鈥渕ap the expansion history鈥� of the universe much further back in time, says Sullivan, potentially ruling out the claims from Lee鈥檚 team.

The exact nature of dark energy, however, remains mysterious. Earlier this year, results from the Dark Energy Spectroscopic Instrument survey indicated that dark energy might not be a constant force, but could vary over time. While that wouldn鈥檛 mean the universe was decelerating right now, it might suggest that the expansion rate has changed over the history of the universe.

鈥淭he needle is pointing a lot more to dark energy being some kind of dynamical thing, not a cosmological constant,鈥� says at Queen Mary University of London. 鈥淓xactly what that is, I think, is a really interesting question.鈥�

Mysteries of the universe: Cheshire, England

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